Stranne
Christian
Stranne
Christian
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ArticleObservations and modeling of a hydrothermal plume in Yellowstone Lake(American Geophysical Union, 2019-05-09) Sohn, Robert A. ; Luttrell, Karen M. ; Shroyer, Emily L. ; Stranne, Christian ; Harris, Robert N. ; Favorito, Julia E.Acoustic Doppler current profiler and conductivity‐temperature‐depth data acquired in Yellowstone Lake reveal the presence of a buoyant plume above the “Deep Hole” hydrothermal system, located southeast of Stevenson Island. Distributed venting in the ~200 × 200‐m hydrothermal field creates a plume with vertical velocities of ~10 cm/s in the mid‐water column. Salinity profiles indicate that during the period of strong summer stratification the plume rises to a neutral buoyancy horizon at ~45‐m depth, corresponding to a ~70‐m rise height, where it generates an anomaly of ~5% (−0.0014 psu) relative to background lake water. We simulate the plume with a numerical model and find that a heat flux of 28 MW reproduces the salinity and vertical velocity observations, corresponding to a mass flux of 1.4 × 103 kg/s. When observational uncertainties are considered, the heat flux could range between 20 to 50 MW.
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PreprintExplosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean( 2007-11-26) Sohn, Robert A. ; Willis, Claire ; Humphris, Susan E. ; Shank, Timothy M. ; Singh, Hanumant ; Edmonds, Henrietta N. ; Kunz, Clayton G. ; Hedman, Ulf ; Helmke, Elisabeth ; Jakuba, Michael V. ; Liljebladh, Bengt ; Linder, Julia ; Murphy, Christopher A. ; Nakamura, Ko-ichi ; Sato, Taichi ; Schlindwein, Vera ; Stranne, Christian ; Tausenfreund, Upchurch ; Winsor, Peter ; Jakobsson, Martin ; Soule, Samuel A.Roughly 60% of the Earth’s outer surface is comprised of oceanic crust formed by volcanic processes at mid-ocean ridges (MORs). Although only a small fraction of this vast volcanic terrain has been visually surveyed and/or sampled, the available evidence suggests that explosive eruptions are rare on MORs, particularly at depths below the critical point for steam (3000 m). A pyroclastic deposit has never been observed on the seafloor below 3000 m, presumably because the volatile content of mid-ocean ridge basalts is generally too low to produce the gas fractions required to fragment a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel Ridge in the Arctic Basin at 85°E, to acquire the first-ever photographic images of ‘zero-age’ volcanic terrain on this remote, ice-covered MOR. Our imagery reveals that the axial valley at 4000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large area greater than 10 km2. At least 13.5 wt% CO2 is required to fragment magma at these depths, which is ~10x greater than the highest values measured to-date in a MOR basalt. These observations raise important questions regarding the accumulation and discharge of magmatic volatiles at ultra-slow spreading rates on the Gakkel Ridge (6- 14 mm yr-1, full-rate), and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global MOR volcanic system.
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ArticleAnalysis and modeling of hydrothermal plume data acquired from the 85°E segment of the Gakkel Ridge(American Geophysical Union, 2010-06-30) Stranne, Christian ; Sohn, Robert A. ; Liljebladh, Bengt ; Nakamura, Ko-ichiWe use data from a CTD plume-mapping campaign conducted during the Arctic Gakkel Vents (AGAVE) expedition in 2007 to constrain the nature of hydrothermal processes on the Gakkel Ridge at 85°E. Thermal and redox potential (Eh) anomalies were detected in two discrete depth intervals: 2400–2800 m (Interval 1) and 3000–3800 m (Interval 2). The spatial and temporal patterns of the signals indicate that the Interval 1 anomalies were most likely generated by a single large, high-temperature (T > 100°C) vent field located on the fault terraces that form the NE axial valley wall. In contrast, the Interval 2 anomalies appear to have been generated by up to 7 spatially distinct vent fields associated with constructional volcanic features on the floor of the axial valley, many of which may be sites of diffuse, low-temperature (T < 10°C) discharge. Numerical simulations of turbulent plumes rising in a weakly stratified Arctic Ocean water column indicate that the high-temperature field on the axial valley wall has a thermal power of ∼1.8 GW, similar to the Trans-Atlantic Geotraverse and Rainbow fields in the Atlantic Ocean, whereas the sites on the axial valley floor have values ranging from 5 to 110 MW.